In heating and melting bulk metals using microwaves, three basic components are generally required: a multimode microwave chamber, a microwave-absorbing crucible, and a thermally insulating casket that is microwave transparent. A metal charge is placed in an open crucible, and the insulating casket is positioned to completely cover the open crucible. The casket and crucible assembly are then placed into a high-power multimode microwave chamber intended to uniformly heat the crucible to the desired temperature when microwave energy is applied to the chamber. The heat absorbed by the crucible from the microwave energy is then able to be transferred to the metal charge. The thermally insulating casket increases the energy efficiency of the microwave system by trapping the heat generated in the crucible. The metal charge in the crucible is quickly heated through radiation, conduction, and convection in the heated crucible. In this way, metal objects that could not be directly heated by microwave energy can be melted easily and efficiently.
To cast the molten metal into a final product, the crucible is often placed over a mold having a desired shape. The metal charge in the crucible is heated until molten. Upon melting, the metal is released and flows into the mold. In order to prevent the metal from solidifying or hardening upon contact with the mold, which could otherwise cause defects such as cavities to be formed in the final result, the mold is heated prior to the flow of metal from the crucible into the mold. Preferably, the metal is cooled and solidifies from the bottom of the mold to the top of the mold to reduce or prevent defects. To accomplish this, a directional temperature gradient is ideally formed in the mold and crucible assembly that promotes cooling of the molten metal from bottom to top. An example of an ideal temperature gradient is shown in FIG. 1. Specifically, a heatable body 106 is provided that includes a crucible 106A and a mold 106B, with hotter areas being represented by darker shading and cooler areas being represented by lighter shading. The crucible 106A has the darkest shading and, therefore, has the highest temperature. Progressing downwards, the temperature gradually falls and the bottom of the mold 106B is at the lowest temperature.
While the above-described directional temperature gradient is known in the prior art, obtaining and maintaining the desired temperature gradient can be difficult for several reasons. In particular, microwaves are preferentially absorbed by whatever absorbs them best. Thus, if two components that absorb microwaves are placed into the same microwave chamber, whichever component absorbs microwaves the best will typically heat much more than the other component. For example, it is possible that a very small component in the system might become superheated and the balance of the system could remain cold. Similarly, if there is arcing or a plasma formation in the chamber, the arc or plasma may absorb essentially all of the energy, which could damage equipment and could result in little energy being imparted to the crucible or mold.
In another example, as the temperature of certain materials (e.g., ceramics) that are used as susceptors in microwave casting increases, their ability to absorb microwaves may change. For purposes of the present disclosure, the word “suscept” means to absorb microwaves to convert the microwaves into heat. Additionally, a material's ability to convert the microwaves into heat will be described as a material's “susceptance level.” A ceramic crucible is a type of susceptor because of its ability to absorb microwaves and to convert them to heat. The fact that susceptance levels of certain materials may be temperature dependent makes microwave heating of those materials (e.g., a ceramic crucible) somewhat unpredictable. There are several known scenarios for heating ceramics. First, the ceramic may be transparent to microwaves, which means it does not absorb microwaves and, therefore, does not heat up in the presence of microwaves. Second, the ceramic might have a greater susceptance level as the temperature of the ceramic increases, which in turn increases its capacity to further absorb microwaves. In other cases, the ceramic's ability to absorb microwaves might decrease as a function of temperature. In such a case, as the ceramic gets hotter, it becomes increasingly more difficult to heat. When using this type of ceramic, it might establish a plateau where it does not get any hotter or it might suddenly drop in temperature once a critical temperature is reached. In still other cases, the ceramic does not start to absorb microwave energy until a critical temperature has been reached. Upon reaching that critical temperature, the ceramic's ability to absorb microwave energy increases as the temperature increases. Lastly, the ceramic may heat in a linear fashion with no change in absorption as a function of temperature.
A problem with microwave casting is that, due to the possible preferential heating of certain components and possible changing physical properties of those components during the heating process, certain portions of the mold and crucible assembly may become too hot or remain too cold. Pouring molten metal under these conditions may be impossible or may result in a less than ideal resulting product.
Correcting the problems using traditional methods are time consuming and can also result in a less than ideal resulting product. For example, as illustrated in FIG. 2, if the crucible 106A is too hot and the mold 106B is too cold, one method of correction is to cut back microwave power. Often, to prevent overheating of the crucible 106A, the power is reduced by 50-75%. This allows the mold to be heated by conduction from the crucible 106A prior to the flow of the molten metal from the crucible 106A to the mold. However, this power reduction significantly increases hold times to heat the mold and, thus, slows the heating process. Multiple rounds of increasing and reducing microwave power may be required to obtain a suitable temperature profile for the crucible and mold, which wastes time and energy. In another example, as illustrated in FIG. 3, if the mold is too hot and the crucible 106A is too cold, a method of correction is to simply pour the metal into the mold as soon as the metal reaches a suitable temperature in the crucible 106A, which may result in defects in the end product. Alternatively, the pour process can be aborted. Again, this is a waste of energy, time, and resources.
What is needed, therefore, is a system and method for controlling the heating and cooling of microwave furnace components that is more efficient and consistent, resulting in a higher quality final product while also reducing energy requirements.